Current- and voltage-controlled oscillators (ICO and VCO) are important components in the structures of transmitters and receivers. When applications to portable wireless communications systems are concerned, the main requirements for VCO/ICOs are: an operational frequency range of 1 to 20 GHz, a very low phase noise and the lowest possible operating voltage and power consumption. Depending on the structure, a communications device may comprise several VCO/ICOs needed for different purposes, e.g. frequency conversion, synthetization, modulation, etc. Their performance affects strongly the performance of the whole communications unit. On the other hand, the demand to implement these oscillators for silicon technologies faces several problems.
Oscillator circuits, i.e. oscillators, can be implemented by many different circuit structures. One of them is an astable (free-running) multivibrator. FIG. 1 shows a conventional emitter-coupled multivibrator circuit, which has been used for implementing voltage-controlled oscillators (VCO). The circuit comprises two transistors Q1 and Q2, between which is provided a positive feedback by cross-coupling each transistor base to the collector of the other transistor. The positive feedback and series resonance circuits Rc1-C and Rc2-C constituted by the resistors RC1 and RC2 and a capacitance C lead to that the output of the multivibrator oscillates continuously between two states, after the oscillation once has been trigged. The oscillation frequency is determined by the component values of the RC series resonance circuits. The oscillation frequency can be controlled by changing some of these component values, typically the capacitance C.
The speed of such a multivibrator circuit (maximum resonance frequency) depends primarily on the properties of the transistors Q1 and Q2. One known way of increasing the speed of the multivibrator circuit is to provide a positive feedback from the collector of one transistor to the base of the other transistor via a buffer transistor. This enables a higher base current, which in turn discharges parasitic capacitances of the base circuit of the transistor faster and accelerates thus the switching of the transistor from one state to another.
The lowest possible operating voltage Vcc is at least 1.1V, when it is assumed that current sources 3 and 4 are ideal, i.e. no voltage loss is provided in them. When the ideal current sources are replaced by some practical circuit structure, such as current mirrors constituted by MOS transistors M11 M21 and M31 in FIG. 2, Vcc increases. The current sources M11, M21 and M31 of FIG. 2 need a direct voltage of about 0.7 V across them, the total Vcc being at least about 1.8 V. The volume resistance of the MOS transistors is the main reason for a high drain source voltage Vds, when the transistors are on. Returning to the operating principle of the circuit, it can be stated that current paths are either Q1-C-M21 or Q2-C-M11 and that the current mirrors produce a stable current through the reference capacitor C, which is the main reason for the typical low phase noise. In search of a new way of increasing the speed, the reference capacitor cannot be decreased much more, because it will be of the order of the parasitic capacitances, which leads to the fact that a controlled planning of the circuit is not possible.
Nowadays there is, however, a need of ever-increasing speeds while an operating voltage as low as possible is desired, especially in electronic equipments using battery power supplies.